1. Field of the Invention
The present invention relates to a method for filtering edible oils, and more particularly, to a method for filtering edible oils utilizing a backwashable filter assembly.
2. Discussion of the Prior Art
The processing of oil for human consumption is a multistep procedure which may include bleaching and hydrogenation. Generally, oils are hydrogenated and bleached in sequential operations. The edible oil is bleached by passing the oil through a bleaching clay such as acid activated calcium bentonite to stabilize the oil and to decolorize the oil in order to prevent oxidation and achieve a clear amber hue to the oil. The edible oil is hydrogenated by reacting the oil with hydrogen in the presence of a catalyst, such as a nickel powder catalyst, to alter the melting point such that the edible oil will solidify at low temperatures, e.g., room temperature. Hydrogenation is not performed for all the edible oils, but rather just those that are intended to be solid at low temperatures. The bleaching and hydrogenation of the edible oils results in the edible oil containing solid particulate matter in the form of bleaching clay and nickel catalyst which must be removed from the edible oil before consumption.
The filtering of edible oils which have been bleached and/or hydrogenated as part of a processing procedure is currently accomplished by pressure leaf filter assemblies having a diatomaceous earth precoat. An exemplary pressure leaf filter assembly, as particularly described in U.S. Pat. No. 3,648,844, comprises a tank containing a stack of closely spaced, generally disk-shaped leaf filters mounted along a central conduit. Each leaf filter may include a plate having two oppositely disposed faces. Porous media are mounted on both faces of the plate, and the plate has passages which communicate between the porous media and the central conduit.
Before the edible oil is introduced into the tank of the leaf filter assembly, a slurry of a precoat material, such as diatomaceous earth, in a liquid is directed into the tank. The liquid passes through the porous media along the passages in the plate to the central conduit and exits the leaf filter assembly via the central conduit. As the liquid passes through the porous media, the precoat material is deposited on the porous media, forming a layer of precoat material on each face of each leaf filter.
Once a sufficient layer of precoat material is deposited on each leaf filter, the flow of slurry into the tank may be terminated and the edible oil containing the particles of nickel catalyst and bleaching clay is introduced into the tank. As the edible oil flows through the precoat layers of each leaf filter, the particles of nickel catalyst and bleaching clay are trapped in the precoat layer. The filtered edible oil then flows through the porous media of the leaf filter along the passages in the plate to the central conduit and exits the leaf filter assembly via the central conduit.
The precoat layer eventually becomes fouled with the particles of nickel catalyst and bleaching clay which are filtered from the oil stream. Essentially, the build-up of particulate solids in the precoat increases the pressure drop across each leaf filter, thereby degrading the performance of the filter. Consequently, the flow of edible oil into the tank of the leaf filter assembly is periodically terminated and the leaf filter assembly is backwashed to remove the precoat layer and the build-up of particulate solids trapped in the precoat layer. Typically, a cleaning liquid, referred to as a backwash liquid, is forced at a high flow rate, pressure, and/or volume in a reverse direction through the central conduit along the passages in the plate of each leaf filter and through the porous media on each face of the plate. The backwash liquid flowing in the reverse direction to the porous medium forces the precoat layer and the particles of nickel catalyst and bleaching clay off of the porous medium and flushes them to the bottom of the tank where the backwash liquid and the solids material is removed via a drain. Once the leaf filter assembly has been backwashed, another cycle of introducing the precoat slurry into the tank to form the precoat layer, introducing the contaminated edible oil into the tank to remove the particles of nickel catalyst and bleaching clay, and backwashing the fouled precoat layer is begun.
While this process of filtering edible oils through a leaf filter assembly is very effective for removing particles of nickel catalyst and bleaching clay, it nonetheless has several problems. For example, this process generates a huge amount of contaminated waste. The volume of precoat material is very large compared to the volume of nickel catalyst and bleaching clay, but all of the solids material must be properly disposed of once it is flushed down the drain of the leaf filter assembly. Further, once the spent precoat layer has been backwashed from the leaf filters, a new precoat layer must again be deposited on each face of each leaf filter. This is a time consuming portion of the cycle which detracts from the overall efficiency of the process since none of the edible oil is being filtered while the precoat layer is being deposited. In addition, because the leaf filters are closely spaced to one another it is difficult to remove all of the precoat from each leaf filter, especially in the area of the leaf filters near the central conduit.
In an alternate design, the leaf filters are stacked on a rotatable hollow shaft. Once the precoat layer on the leaf filters becomes fouled, filtration may be suspended and the shaft is rotated to remove the caked on particulate matter. The shaft is rotated at a speed sufficient to generate a centrifugal force which causes the caked on matter to fly off the leaf filters. Accordingly, as with the backwashing method described above, the rotation not only serves to remove the caked on matter, but also the diatomaceous precoat as well.
In accordance with one aspect, the present invention is directed to a method for filtering edible oils. The method of filtering edible oils comprises passing edible oil containing solid particulate matter into a backwashable filter assembly, removing the solid particulate matter from the edible oil by directing the edible oil through a fiber filter medium having a graded pore structure, including passing the edible oil through a coarser pore upstream region and then a finer pore downstream region of at least one substantially cylindrical filter element contained within the backwashable filter assembly and accumulating the solid particulate matter directly on the at least one substantially cylindrical filter element. The method of filtering edible oils also comprises backwashing the at least one substantially cylindrical filter element after the cake of solid particulate matter accumulates on the at least one substantially cylindrical filter element by directing a backwash fluid through the fibrous filter medium having a graded pore structure, including passing the backwash fluid through the finer pore downstream region and then the coarser pore upstream region, to clean the at least one substantially cylindrical filter element having the solid particulate matter removed from the edible oil accumulated thereon. The method also includes cyclically alternating between removing the solid particulate matter from the edible oil by passing the edible oil through the at least one substantially cylindrical filter element and backwashing the at least one substantially cylindrical filter element to clean the at least one substantially cylindrical filter element having solid particulate matter accumulated thereon.
In accordance with another aspect, the present invention is directed to a method for filtering edible oils. The method comprises passing a fluid containing solid particulate matter into a first backwashable filter assembly and into a second backwashable filter assembly, removing the solid particulate matter from the fluid by directing the fluid through at least one filter element in the first backwashable filter assembly, including accumulating a cake of solid particulate matter on the at least one filter element in the first backwashable filter assembly, and through at least one filter element in the second backwashable filter assembly, including accumulating a cake of the solid particulate matter on the at least one filter element in the second backwashable filter assembly. The at least one filter element in the first backwashable filter assembly and the at least one filter element of the second backwashable filter assembly comprise a fibrous filter medium having a graded pore structure. The edible oil is passed through a coarser pore upstream region and then a finer downstream region. The method also comprises backwashing the at least one filter element in the first backwashable filter assembly after the cake of solid particulate matter is formed on the at least one filter element of the second backwashable filter assembly, and backwashing the at least one filter element in the second backwashable filter assembly after the cake of solid particulate matter is formed on the at least one filter element of the first backwashable filter assembly.
In accordance with another aspect, the present invention is directed to a method of filtering edible oil fluids. The method of filtering edible oil fluids comprises passing an edible oil fluid containing solid particulate matter into an inlet of a first backwashable filter assembly, removing the solid particulate matter from the fluid by directing the fluid through the at least one filter element in the first backwashable filter assembly, including building a cake of the solid particulate matter on the at least one filter element in the first backwashable filter assembly, discontinuing passing of the fluid containing solid particulate matter into the inlet of the first backwashable filter assembly, passing the fluid containing solid particulate matter into an inlet of a second backwashable filter assembly through an outlet of the second backwashable filter assembly and into the inlet of the first backwashable filter assembly, removing the solid particulate matter from the fluid by directing the fluid through the at least one filter element in the second backwashable filter assembly, including accumulating a cake of the solid particulate matter on the at least one filter element in the second backwashable filter assembly and through the at least one filter element in the first backwashable filter assembly, and through the cake of the solid particulate matter on the at least one filter element in the first backwashable filter assembly, and finally discontinuing passing the fluid from the outlet of the second backwashable filter assembly to the inlet of the first backwashable filter assembly. The method also comprises backwashing the at least one filter element in the first backwashable filter assembly, discontinuing passing of the fluid containing solid particulate matter into the inlet of the second backwashable filter assembly, passing the fluid containing solid particulate matter into an inlet of the first backwashable assembly through the outlet of the first backwashable filter assembly and into the inlet of the second backwashable filter assembly, removing the solid particulate matter from the fluid by directing the fluid through the at least one filter element in the first backwashable filter assembly, including accumulating a cake of the solid particulate matter on the at least one filter element in the first backwashable filter assembly, and through the at least one filter element in the second backwashable filter assembly, including accumulating a cake of the solid particulate matter on the at least one filter element in the second backwashable filter assembly, discontinuing passing the fluid from the outlet of the first backwashable filter assembly, and backwashing the at least one filter element in the second backwashable filter assembly.